I am almost entriely sure that there is a cap for maxing block and that the cap is 75%. What this means is that when wearing a Guardian angel, you aimn you dextery and shield to yield 55 and GA will take care of the rest. Generally, this means that you need to invest less points in Dextery.

There's a formula, based on shield block % and amount of Dex, that will tell you how much you need at a given level... But I haven't memorized it >_< On average, with a good shield, you need about 2 Dex for 1% block, I think.

This calculation is done for you when your Chance to Block displays as hovertext over your Defense Rating (as alainpp66 said). If you feel like doing it yourself, just remember that the Blocking variable includes both your Shield Block % as well as any +% Block gear you have (such as Guardian Angel).